This invention relates generally to semiconductor technology, and more particularly, the invention relates to junction-isolated bipolar transistor structures fabricated in silicon semiconductor material.
Bipolar transistor devices and circuits are often fabricated in junction-isolated tubs in a silicon wafer to provide electrical isolation between circuit components, particularly in linear integrated circuits. Typically, an epitaxial layer of one conductivity type is grown on a silicon substrate of opposite conductivity type. The device tub is then defined in the epitaxial layer by forming a doped barrier of opposite conductivity type around the tub and extending from the surface of the epitaxial layer to the substrate. The doped barrier is formed by doping a shallow region in the epitaxial layer surface and then diffusing the dopants to the underlying substrate by thermal drive-in. The thermal drive-in also diffuses the dopants laterally so that the barrier assumes a generally semicircular cross section. This unwanted lateral diffusion reduces the available surface area for the integrated circuit components fabricated in the individual isolated tubs.
Heretofore, an attempt has been made to reduce the cross section of the barrier region by forming a highly doped region in the substrate prior to the epitaxial layer growth, and thereafter up-diffusing dopant from the substrate while dopant is down-diffused from the surface of the epitaxial layer. Accordingly, the dopant need diffuse only half the thickness of the epitaxial layer and, consequently, the lateral diffusion is also reduced, resulting in a larger tub surface. A problem with this approach is the alignment of the two doped regions so that the up-diffusion and the down-diffusion of dopants will meet.